This invention relates to a method and an apparatus for controlling the voltage magnitude and reactive power at a power plant and/or power substation connected to an electric power system.
The increase of power demand has constrained the growth of power generation to precede, so that a modern electric power system has been steadily advancing in size and capacity. In addition, transmission lines have extraordinarily increased in number and length, since a multiplicity of power plants and load centers have been scattered all over the service area.
As well known, an electric power system is an aggregation of power facilities for generation, transmission and distribution, in which various types of power plants such as hydraulic, thermal and nuclear are located in disorder and interconnected in complexity. This is one material reason why a modern power system has been possessed by the complicated characteristics.
The operation of a modern power system, it is apparently believed, has been successfully carried out by the aid of signal facilities for communication, telemetering, supervisory, remote control, protective relaying and automatic control, in which controls of generation, frequency, voltage and power flow are included.
The generation control, or the demand and supply operation of a power system, performs an important function in a rough regulation of the system frequency, by which the generation of power plants is adjusted in accordance with ever-changing load conditions so as to maintain the system frequency within a prescribed range.
Fine regulation of the system frequency is carried out mainly by speed governors with which most of hydraulic and thermal power plants are equipped. According to a recommendation, frequency variations whose period lies between 2 and 20 minutes, might be absorbed by the so-called load-frequency control plants, and further, frequency deviations lasting for more than 20 minutes should be taken care of by the so-called load-dispatch control plants.
As shown in "Standard Handbook for Electrical Engineers," Donald G. Fink, H. Wayne Beaty, eleventh edition, 16-2-16-47, power flow control is an essential need for the prevention against overload of power equipments, the reduction of transmission losses and the control of system voltage, including the regulation of an active and a reactive power at individual power plants, the modification of network by switching of power plants and substations as well as transmission lines, and the regulation of reactive power at phase-modifying equipments or reactive power suppliers.
As shown in "Standard Handbook for Electrical Engineers", 14-33-14-37, control of the system voltage is carried out mainly by automatic voltage regulators with which most of power plants and substations are equipped, while supplementarily by power-flow control system varying the reactive power output of Var-compensating equipments such as power capacitors, shunt reactors, on-load tap-changing transformers and synchronous capacitors.
All of the above-mentioned controls are exercised in compliance with informations furnished by the characteristics of frequency and voltage variations as well as decisions derived from the nature of loads and the experience of operations, which have been based on the past records at suitable nodes of the power system.
In general, the load dispatch control referred to as generation allocation is esteemed to act prior to other controls. For that reason, considerable attentions have so far devoted to the control of active power, but not to the controls of voltage and reactive power regardless of their importance to system operations.
Actually, as shown in the above described "Standard Handbook for Electrical Engineers," 14-16-14-33, untolerable difficulties in the operation of a modern power system may be derived from the lack of considerations on reactive power and voltage control channel. Furthermore, in transmission networks every node voltage is affected more severely by reactive power flows than by active power flows and the magnitude of voltage variation depends dominantly on the network impedances.
It is quite necessary for system operations, therefore, to exactly grasp the voltage and reactive power characteristics.
The reactive power output of a phase-modifying equipment depends on the location where it is installed in the network, and the condition under which it is interconnected with power plants, substations and other phase-modifying equipments.
The so-called matrix equation is particularly useful in calculating the power flows in a linear impedance network. But because of matrix whose dimension increases along with the number of nodes in the network, a large-scale power system is restricted by dimension in power-flow calculations even by means of digital computers. Furthermore, actual power systems are provided with nonlinear characteristics to make the system operations inconvenient. Especially, fluctuations in node voltages are injurious to power-flow calculations based on matrix equations, whereas for the sake of simplification in system operations the so-called constant power-factor operation is inevitablly applied to small-capacity hydraulic power plants which are subject to variations of the system voltage and provided with the amount of reactive power not enough to regulate the system voltage.
The objective of power-flow calculations is to analyze the power transfers in a transmission network which is expressed in terms of impedance or admittance. Until the late 1950s ac network analyzers were much used as analog models for solving power-flow problems, but because of the imperfectness of the analog simulation any useful results could be scarcely obtained. In the 1960s digital computers were placed at the service of power-system analyses, but the formulation of power-system performances remains as incomplete as ever, which includes the characteristic equations of generator units equipped with speed-governing and voltage-regulating systems.
Power-system stability is an important problem for system engineers to solve, which is associated not only with the loss of a large-capacity power plant forcing other power plants and thus the power system to overload but also with the leading-phase operation of thermal power plants causing themselves to be in unstable conditions.
There is a method of solving steady-state stability limits in terms of phase angles between terminal voltages of lines, internal voltages of machines or an internal voltage and a terminal voltage. The solutions, however, would be substantially incorrect because of neglecting the effects of control actions on generator performances. Besides, it might be quite impractical to analyze the dynamical stability of a voltage-regulated power plant operating in parallel with an infinite bus.
On the whole, theoretical approaches to power-system performances have so far been made on condition that the relations between power-system components are simplified and the operating characteristics of individual components are linearized. All of the results, therefore, should be inapplicable to actual power systems characterized essentially by large-scale and complexity.